1. Field of the Invention
The present invention relates to a method of preventing polymer scale deposition on the inner wall and so forth of a polymerization vessel during polymerization of a monomer having an ethylenically double bond.
2. Description of Prior Art
In processes of preparing polymers by polymerizing a monomer in a polymerization vessel, the problem that polymers deposit on the inner wall and other parts which come into contact with the monomer such as stirring blades of the polymerization vessel in the form of scale, is known. The deposition of the polymer scale on the inner wall results in disadvantages that the yield of the polymer and the cooling capacity of the polymerization vessel are lowered; that the polymer scale may peel and mix into manufactured polymer, thereby impairing the quality of the manufactured polymer; and that removal of such polymer scale is laborious and hence time-consuming. Further, since the polymer scale contains unreacted monomer, there is a danger that the workers may be exposed to the unreacted monomer, which may give them some physical disorders.
Heretofore, there are known methods for preventing polymer scale deposition in which the inner wall and so forth are coated with various materials that act as polymer scale preventive agents, for example, particular polar compounds (Japanese Patent Publication (KOKOKU) No. 30343/1970); dyes or pigments (Japanese Patent Publication (KOKOKU) Nos. 30343/1970 and 30835/1970); an aromatic amine compound (Japanese Pre-examination Patent Publication (KOKAI) No. 50887/1976); a reaction product of a phenolic compound with an aromatic aldehyde (Japanese Pre-examination Patent Publication (KOKAI) No. 54317/1980).
These methods are effective in preventing polymer scale deposition, in the case where the monomer to be polymerized is vinyl halide such as vinyl chloride or a monomer mixture containing a large amount of vinyl halide and a small amount of other monomers copolymerizable therewith. However, the scale-preventing effect achieved varies depending on polymerization conditions such as the kind of monomers, the kind of a polymerization catalyst, form of polymerization system and the material of the inner wall of a polymerization vessel; it is difficult to prevent the scale deposition effectively and certainly. Specifically, when a polymerization catalyst with a strong oxidative effect such as potassium persulfate, acetylcyclohexylsulfonyl peroxide and di-2-ethoxyethyl peroxydicarbonate is used, the scale preventive agent forming the coating may be oxidized, so that the scale preventing effect is impaired. In the case of polymerization vessels with inner wall made of stainless steel or other steels, polymer scale deposition is liable to occur as compared with vessels lined with glass. Emulsion polymerization is liable to form polymer scale deposition as compared with suspension polymerization.
Of the scale preventive agents used in the above methods, the dyes and pigments described in Japanese Patent Publication (KOKOKU) No. 30835/1970 and 24953/1977, the aromatic amine compounds described in Japanese Pre-examination Patent Publication (KOKAI) No. 50887/1976, the reaction products of a phenolic compound with an aromatic aldehyde described in Japanese Pre-examination Patent Publication (KOKAI) No. 54317/1980 are colored and therefore possibly have a disadvantage of putting a color to product polymers.
Aniline, nitrobenzene, formaldehyde, etc. of the polar organic compounds described in Japanese Patent Publication (KOKOKU) No. 30343/1970, and the pigments containing a heavy metal such as chromium and lead described in Japanese Patent Publication (KOKOKU) No. 30835/1970 are poisonous. The dyes described in Japanese Patent Publication (KOKOKU) Nos. 30835/1970 and 24953/1977 include dyes that involve some concern about carcinogenesis. These substances may cause serious problems from viewpoint of safety in work operations.